There are a myriad of prior art references which teach that polycarbonates alone or in combination with other polymers can be stabilized by conventional stabilizers. However, the only specific disclosure of the stabilization of blends of polycarbonate and the styrenic polymer ABS (acrylonitrile-butadiene-stryene) is by an anonymous author in Research Disclosure, October 1993, pp 673-675 where the use of a combination of UV absorber, a hindered amine and other additives is touted. There is no mention that the hindered amine should be non-basic.
Polymer blends provide a valuable balance of mechanical properties, melt processing and cost. Blends have gained wide industrial acceptance as a result. One example is polycarbonate (PC) blended with rubber modified styrenics such as poly(acrylonitrile-butadiene-styrene) (hereafter ABS). The PC provides high mechanical and thermal properties while the ABS allows for a lower processing temperature (than for neat PC) and imparts enhanced low temperature impact strength as seen in the prior art. PC blends are widely used in demanding applications such as automotive parts, power tools, and business equipment housings. In particular, automotive specifications are stringent regarding color retention (lack of discoloration) for PC/ABS and PC/PBT blends. The General Motors Engineering Materials and Processes PLASTICS Standards June 1995, for colored PC/ABS requires a color change test value of delta E equal to 3.0 maximum after 263 kJ/m.sup.2 in an interior Xenon Arc apparatus.
However, adequate stabilization of PC blends against terrestrial light remains problematic. The current state of the art is the use of a UV absorber only. A high concentration of such UV absorber is often needed resulting in high cost for such PC blends. While the UV absorber is beneficial in screening UV light, the UV absorber does not act to scavenge radical species which arise from UV induced or thermal induced degradation of the PC or the ABS components in the blends. The hindered amines are well known to be radical scavengers for many classes of polymeric materials such as polyolefins, rubber and styrenic copolymers and terpolymers, including ABS as taught by P. M. Klemchuk et al., Makromol. Chem., Makromol Sympos. 28, 117 (1989).
However, hindered amines cannot be used in polymeric substrates which are sensitive to chemical reaction with the basic nitrogen atom in the hindered amine as taught by G. L. Gaines, Jr. Polymer Degradation and Stability, 27, 13 (1990). Such polymeric substrates include PC and possibly polyesters. Indeed, a severe reduction in mechanical properties is reported when a state-of-the-art hindered amine [bis(2,2,6,6-tetramethyl-piperidin-4-yl) sebacate, TINUVIN.RTM. 770, Ciba] is incorporated into a PC/ABS blend as taught by M. Clauss et al.,"Stabilization of PC/ABS blends with a polymer bound hindered amine light stabilizer", Additives 95 meeting, Feb. 22-24, 1995. The PC/ABS blends are susceptible to loss of useful mechanical properties after only a few days of exposure to accelerated weathering conditions. This loss of properties is reported to be due in part to UV induced photooxidation of the polybutadiene portion of the ABS as taught by J. D. Cooney, Polymer Engineering and Science, 22, 492 (1982) and J-L. Gaudette et al., Polymer Degradation and Stability, 48, 457 (1995). While it is known that ABS can be stabilized against terrestrial light by the use of hindered amine or hindered amine in combination with a UV absorber, such hindered amine cannot be used to stabilize the ABS when polycarbonate is present in a multicomponent polymer blend for reasons discussed above. Thus, the PC blends industry is still in need of suitable hindered amines which exhibit no deleterious reaction with the polymers in the blend.
Surprisingly, there is a certain group of hindered amines which appears to be non-interacting in a deleterious manner, and hence suitable for polycarbonate blends. The hindered amines based on 3,3,5,5-tetramethyl-2-piperazinone type structures (GOODRITE.RTM. compounds) demonstrate superior efficacy as stabilizers against UV light in PC/ABS blends. These GOODRITE.RTM. type hindered amines alone are not sufficient to provide adequate stabilization of the polymer blends against UV light. However, the combination of a UV absorber with these hindered amines does provide adequate stabilization against UV light for such polymer blends. Notably absent in all the prior art is any discussion of the use of combinations of such a hindered amines with a UV absorber to stabilize a polymer or a polymer blend.
U.S. Pat. No. 4,753,979 teaches the use of a 4-acyl-2-piperazinone type compound for stabilization of a range of polymer substrates. U.S. Pat. Nos. 4,240,961; 4,292,240; 4,480,092; 4,722,806; 5,071,981 and 5,098,944; and PCT WO 92/04407 describe the GOODRITE.RTM. type hindered amines. These stabilizers are primarily taught as useful in polyolefins, polyacetals (U.S. Pat. No. 5,059,644) and polyglutarimides (EP 577,292). There is no specific teaching that they can or should be used with a UV absorber for stabilizing a polymer or a polymer blend.